wic: move sdimage-raspberrypi to toplevel wic location

Wic supports picking up image files from toplevel LAYERDIR/wic directory. Using
this location has the benefit that image files are easier to find (compare that
to previously used scripts/lib/image/canned-wks/ location).

Signed-off-by: Maciej Borzecki <maciej.borzecki@rndity.com>